Oxygen is a commodity chemical in the industrial gas industry. It has numerous applications including waste water treatment, glass melting furnaces, and the steel industry. One of the most common methods of oxygen production is by cryogenic distillation of air. However, this technology is not competitive for small size oxygen plants (&lt;100 TPD O.sub.2). The technology of choice for this size range is adsorption. There is a need in the marketplace to produce oxygen at low capital and energy costs by adsorptive gas separation.
Adsorptive processes are extensively used in the industry to produce oxygen from air for small size oxygen plants (&lt;100 TPD O.sub.2). There are two major categories of these processes--pressure swing adsorption processes (PSA) and vacuum swing adsorption processes (VSA). The pressure swing adsorption processes carry out the adsorption (feed) step at pressures much higher than ambient and adsorbent regeneration at pressures close to ambient. The adsorbent beds go through secondary process steps, such as pressure equalizations, depressurizations, blowdowns, and purge or various combinations of these during the cycle. Typical of the O.sub.2 -PSA processes are U.S. Pat. Nos. 3,430,418; 4,589,888; 4,650,501 and 4,981,499.
Primary reasons for high energy consumption in PSA processes are: (1) O.sub.2 recovery from these processes is low, and (2) the entire feed stream has to be compressed up to the adsorption pressure. These inefficiencies are somewhat circumvented in vacuum swing adsorption (VSA) processes. In these processes, adsorption is carried out at pressure close to ambient and adsorbent regeneration is carried out at sub-atmospheric levels. The adsorbent beds go through several secondary steps with the primary aim of increasing oxygen recovery and reducing adsorbent inventory per unit of product gas.
U.S. Pat. No. 3,636,679 discloses an adsorptive process which cocurrently depressurizes to provide simultaneous purge and product. The process does not include product production while feed gas is administered to the adsorptive bed.
U.S. Pat. No. 3,717,974 also discloses cocurrent depressurization to provide simultaneous purge and product. Such a process step is disadvantageous because it uses high purity cocurrent depressurization gas for purge even at the initial depressurization.
U.S. Pat. No. 5,122,164 describes an O.sub.2 VSA process with the steps: adsorption, simultaneous cocurrent depressurization and countercurrent evacuation, countercurrent evacuation, vacuum purge, pressure equalization with gas from a bed undergoing cocurrent depressurization and product repressurization.
U.S. Pat. No. 5,223,004 describes an O.sub.2 VSA process with the steps: adsorption, simultaneous cocurrent depressurization and countercurrent evacuation, countercurrent evacuation, purge, repressurization with product and cocurrent depressurization gas from another bed and repressurization with product and feed.
U.S. Pat. No. 5,246,676 describes a process for producing oxygen from air containing at least three beds and undergoing the following steps: adsorption, countercurrent evacuation including at least two successive pumping sub-steps, and product repressurization. Various options on the cycle include; cocurrent depressurization to provide purge gas to the bed under vacuum, further cocurrent depressurization to provide partial repressurization gas, and cocurrent depressurization to a storage tank from where some of the purge gas is withdrawn.
French Patent W093/10882 describes an oxygen VSA process which incorporates two stages of evacuation. Each stage is carried out by a separate vacuum machine. These machines could be of the same type or different type, e.g. volumetric and centrifugal.
Despite the prior art, a need still exists for an O.sub.2 VSA process with higher oxygen recovery (i.e., lower energy costs) and lower adsorbent requirement per unit of oxygen production (i.e., lower capital costs) than the current processes. The present invention outlines a vacuum swing adsorption (VSA) process to produce oxygen from air at higher oxygen recovery and lower adsorbent requirement per unit of oxygen product than current O.sub.2 VSA processes.